Light emitting diodes (LEDs) are semiconductor devices which send and receive a signal by converting electricity into infrared light or visible light using characteristics of compound semiconductors or which are used as light sources.
Group III-V nitride semiconductors receive much attention as essential materials for light emitting devices such as light emitting diodes (LEDs) or laser diodes (LDs) due to physical and chemical properties thereof.
Such a light emitting diode does not contain environmentally harmful substances such as mercury (Hg) used in conventional lighting apparatuses such as incandescent lamps or fluorescent lamps and thus advantageously has superior eco-friendliness, long lifespan and low power consumption, thus being used as alternatives of conventional light sources.
FIG. 1 is a sectional view of an LED having a general flip bonding structure.
The LED shown FIG. 1 includes a submount 10, a passivation layer 12, first and second electrode pads 14 and 16, bumps 18, 20 and 22, first and second electrode layers 24 and 26, a semiconductor layer 30, an AlN layer 40 and a sapphire substrate 42. The semiconductor layer 30 includes a p-type semiconductor layer 32, an active layer 34 and an n-type semiconductor layer 36.
In the LED shown in FIG. 1, light emitted from the active layer 34 passes through the n-type semiconductor layer 36 and the AlN layer 40 and then exits through the sapphire substrate 42 upwardly. At this time, in accordance with Snell's Law, due to a difference in index of refraction between the n-type semiconductor layer 36, the AlN layer 40 and the sapphire substrate 42, a part 2 of light emitted from the active layer 34 does not escape from the sapphire substrate 42, undergoes total internal reflection and is absorbed in the semiconductor layer 30, thus causing deterioration in luminous efficacy.
When the LED shown in FIG. 1 is a blue LED emitting a blue wavelength band of light, the AlN layer 40 may be omitted and the n-type semiconductor layer 36 may be formed of GaN. However, when the LED shown in FIG. 1 is a DUV LED emitting a deep ultraviolet (DUV) wavelength band of light, the AlN layer 40 is formed and the n-type semiconductor layer 36 is formed of AlGaN. The AlN has an index of refraction of 2.3, the sapphire substrate 42 has an index of refraction of 1.82 and the air contacting the sapphire substrate 42 has an index of refraction of 1. Accordingly, a difference in index of refraction between media present in a light passage greatly increases, disadvantageously, total internal reflection loss is maximized and light extraction efficiency is thus deteriorated.
FIG. 2 is a view comparing a dose of light which exits from a side surface of a sapphire substrate 50 in case of a blue LED and a DUV LED, wherein ΘA represents an angle of incidence and ΘB represents an angle of refraction.
The reference numeral ‘52’ of FIG. 2 corresponds to the GaN buffer layer 40 or the GaN light emitting structure 36 in the case of the blue LED, and corresponds to the AlN layer 40 in the case of the DUV LED. In this case, assuming that a wavelength (λ) of light emitted from the blue LED is 450 nm and a wavelength (λ) of light emitted from the DUV LED is 280 nm, indexes of refraction of the respective layers 50 and 52 are shown in the following Table 1.
TABLE 1Index of refractionIndex of refractionTypes(λ = 280 nm)(λ = 450 nm)AlN2.312.18GaN2.712.48Sapphire1.821.78
Also, a total internal reflection angle ΘTIR, an angle of incidence ΘA and an angle of refraction ΘB of a blue LED having a wavelength (λ) of 450 nm and a DUV LED having a wavelength (λ) of 280 nm are shown in the following Table 2.
TABLE 2θB (°)Typesλ (nm)θTIR (°)θA = 15°θA = 30°θA = 40°AlN/Sapphire280 nm52.47193954boundarySapphire/air33.24boundaryGaN/Sapphire450 nm45.86214464boundarySapphire/air34.18boundary
It can be seen from Tables 1 and 2 that, since, in the same angle of incidence ΘA, an angle of refraction ΘB at an GaN/sapphire boundary having a wavelength of 450 nm is larger than an angle of refraction ΘB at an AlN/sapphire boundary having a wavelength of 280 nm, light escaping from the GaN/sapphire boundary having a wavelength of 450 nm progresses near a horizontal plane. However, since the total internal reflection critical angle (ΘTIR), at AlN/sapphire boundary having a wavelength of 280 nm is larger than the total internal reflection critical angle (ΘTIR) at GaN/sapphire boundary having a wavelength of 450 nm, light of 6.61° (=52.47−45.86) corresponding to a critical angle difference is emitted in a greater amount from the 280 nm AlN/sapphire boundary through the sapphire substrate 50, as compared to the 450 nm GaN/sapphire boundary, when light is uniformly radiated in a spherical shape at all angles from the active layer 34 (MQW) of the LED. Accordingly, a dose of light which exits from a lower part of a sidewall of the sapphire substrate 50 in the DUV LED is greater than that in the blue LED.
As such, as compared with the blue LED, the DUV LED may exhibit deteriorated light extraction efficiency, since a great amount of light emitted from the active layer 34 may exit to the lower part of the sidewall of the sapphire substrate 42.